The present invention relates to a developing solution and a pattern forming method using the same, for forming patterns on a semiconductor substrate surface or the like.
The development of high speed discrete device using a compound semiconductor has been promoted recently. In order to enhance the characteristics of such device and realize faster operation, it is necessary to narrow the width of the opening for forming the gate electrode. When the opening width becomes narrower, however, the gate electrode resistance increases, which is known to hinder the speed from increasing.
To solve this problem it has been conventionally known to use a T-shaped gate electrode as the gate electrode. The T-shaped gate electrode is so structured as to be narrower at the contact portion and wider in the upper portion of the electrode.
FIGS. 10(a) to (f) show a process sectional view for forming a conventional GaAs FET T-shaped gate electrode by employing the electron beam exposure and liftoff technology.
In FIG. 10(a), numeral 1 shows a GaAs substrate consisting of an epitaxial layer of GaAs, or GaAs and AlGaAs functioning active layer on semi-insulating GaAs. A source and drain electrodes 2 made of AuGe/Ni are formed on a GaAs substrate 1, and an SiO.sub.2 spacer 3 is formed on the GaAs substrate 1. Next, on the source, drain electrodes 2, SiO.sub.2 spacer 3 and GaAs substrate 1, a PMMA (polymethyl methacrylate) resist layer 4 with a mean molecular weight of 600,000 is applied, and is heated. In succession, on the PMMA resist film 4, a PMMA resist film 5 with a mean molecular weight of 200,000 is applied and is heated. Next, a specified portion is exposed by using a 50 kV electron beam 6.
Then, using MIBK (methyl isobutyl ketone), the PMMA resist films 4, 5 are developed, and a PMMA resist pattern 7 is formed (FIG. 10(b)).
In FIG. 10(c), using the PMMA resist pattern 7 as the mask, the SiO.sub.2 spacer 3 is etched by a wet process so as to form an SiO.sub.2 spacer pattern 3a.
In FIG. 10(d), using the SiO.sub.2 spacer pattern 3a as the mask, the GaAs substrate 1 is etched in wet process by using a mixed aqueous solution of tartaric acid and hydrogen peroxide as an etchant, and a recess structure 8 is formed.
Consequently, an Al film 9 is evaporated on the surface of PMMA resist films 4, 5 and GaAs substrate 1, and an Al gate electrode, that is, a T-shaped gate electrode 10 is formed (FIG. 10(e)).
Further, dissolving and removing the PMMA resist films 4, 5 with organic solvent or the like, the unnecessary Al film 9 is lifted off, and the structure in FIG. 10(f) is obtained.
In order to obtain the T-shaped gate electrode 10, in FIG. 10, an example of using PMMA resist films 4, 5 is shown, but since the PMMA resist films 4, 5 are inferior in heat resistance and adhesion to the substrate, a process of using PMGI resist or the like has been developed in order to improve these points. In the process using the PMGI resist, an alkaline aqueous solution, or a mixed aqueous solution of alkali and salt is used as the developing solution. In this case, as the alkali, tetramethyl ammonium hydroxide is widely used. The salt acts as surfactant, and usually tetraalkyl ammonium salt is used.
In the prior art, however, when the PMGI resist is developed by an alkaline aqueous solution or a mixed aqueous solution of alkali and salt, the change of rate of dissolution to the change of exposure dose is slow, or the normalized remaining film thickness in the unexposed portion and the gamma value are small. Accordingly, it is difficult to form a pattern with a steep side wall. Even when a single layer of PMGI resist is used, it is difficult to form the pattern in a size of 0.4 .mu.m or less. It is hence impossible to use it for forming T-shaped gate electrode in a fine pattern. Or when the concentration of the tetramethyl ammonium hydroxide is diluted before use, the remaining film thickness change due to the change of exposure dose becomes larger, that is, the gamma value becomes higher, but the sensitivity is lowered. Besides, as a result of dilution of tetramethyl ammonium hydroxide, the hydrophilic property is enhanced, and the wettability to the hydrophobic resist becomes poor. Accordingly, when developing the PMGI resist used as the lower layer of the two-layer resist, if the opening width of the upper layer resist pattern is narrow, the developing solution of the PMGI resist does not flow smoothly into the opening, and the PMGI resist can not be developed evenly. To avoid this, it is considered to enhance the wettability by mixing the developing solution with surfactants, but in this case, the gamma value becomes smaller, and the resolution is lowered.
In the process shown in FIG. 10, since the PMMA resist film 5 with the mean molecular weight of 200,000 in the upper layer and the PMMA resist layer 4 with the mean molecular weight of 600,000 in the lower layer are developed simultaneously, it is difficult to control and optimize the opening width of the upper resist and that of the lower resist at the same time. Still futher, since the adhesion of the PMMA resist film 4 to the GaAs substrate 1 is poor, it is necessary to place the SiO.sub.2 spacer 3. If the adhesion is poor, the SiO.sub.2 spacer 3 at the edge portion 7a (FIG. 10(b)) of the PMMA resist pattern 7 and the bottom of the PMMA resist film 4 are lifted, and therefore in the wet etching process of the pattern of the SiO.sub.2 spacer 3, the etchant may reach as far as the bottom of the edge portion 7a of the PMMA resist pattern 7, and therefore the SiO.sub.2 spacer pattern 3a may be wider than desired. Since the SiO.sub.2 spacer 3 is isotropically etched in the wet etching process, the controllability of the opening width is poor, and the opening width tends to be wider than the opening width of the PMMA resist mask. Accordingly, the opening width of the recess structure 8 becomes broader. Besides, the distance between the bottom of the lower PMMA resist film 4 and the bottom of the recess structure 8 is extended by the portion of the thickness of the SiO.sub.2 spacer 3, and the evaporated T-shaped gate electrode 10 is easily separated at the position indicated by part A in FIG. 10(f). That is, the width of the evaporated T-shaped gate electrode 10 is defined by the opening width of the lower PMMA resist film 4, and is deposited, but the Al is also evaporated on the side wall at the opening of the lower PMMA resist film 4, and the opening width of the lower PMMA resist film 4 becomes narrower as the evaporation time passes, and the evaporated deposit becomes triangular. Finally, the opening is plugged by the evaporation deposit collected at both side walls of the opening of the lower PMMA resist film 4, and if evaporation is further continued, Al is not evaporated at all in the gate area. That is, as compared with the opening width of the lower PMMA resist film 4, the distance from the substrate surface for depositing the evaporation deposit to the opening of the lower PMMA resist film 4 is longer, and in this state, when the T-shaped gate electrode 10 is deposited, it is formed like a mushroom from the part nearer to the upper end of the triangle, and the umbrella-shaped portion and the root of the stem of the mushroom are likely to be broken or peeled off when lifting off. It is therefore difficult to form the T-shaped gate electrode with a narrow gate length. If the SiO.sub.2 spacer 3 is not installed, in the wet etching process of the GaAs substrate, the etchant may invade into the interface of the resist film and GaAs substrate, which may further extend the width of the recess structure.